Add LintDuplicateMetric to promlint

Signed-off-by: Hrishikesh Barman <hrishikeshbman@gmail.com>

prometheus/testutil/promlint/promlint_test.go

@@ -839,3 +839,27 @@ mc_something_total 10
 		lintAndVerify(l2, cv)
 	})
 }
+
+func TestLintDuplicateMetric(t *testing.T) {
+	const msg = "metric not unique"
+
+	tests := []test{
+		{
+			name: "metric not unique",
+			in: `
+# HELP not_unique_total the helptext
+# TYPE not_unique_total counter
+not_unique_total{bar="abc", spam="xyz"} 1
+not_unique_total{bar="abc", spam="xyz"} 2
+`,
+			problems: []promlint.Problem{
+				{
+					Metric: "not_unique_total",
+					Text:   msg,
+				},
+			},
+		},
+	}
+
+	runTests(t, tests)
+}

prometheus/testutil/promlint/validation.go

@@ -30,4 +30,5 @@ var defaultValidations = []Validation{
 	validations.LintReservedChars,
 	validations.LintCamelCase,
 	validations.LintUnitAbbreviations,
+	validations.LintDuplicateMetric,
 }

prometheus/testutil/promlint/validations/duplicate_validations.go

@@ -0,0 +1,37 @@
+// Copyright 2024 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package validations
+
+import (
+	"fmt"
+	"reflect"
+
+	dto "github.com/prometheus/client_model/go"
+)
+
+// LintDuplicateMetric detects duplicate metric.
+func LintDuplicateMetric(mf *dto.MetricFamily) []error {
+	var problems []error
+
+	for i, m := range mf.Metric {
+		for _, k := range mf.Metric[i+1:] {
+			if reflect.DeepEqual(m.Label, k.Label) {
+				problems = append(problems, fmt.Errorf("metric not unique"))
+				break
+			}
+		}
+	}
+
+	return problems
+}